It has become increasingly desirable to increase the efficiency and reduce the size of power-producing or thrust-producing devices such as gas turbine engines. Gas turbine engines typically include one or more shafts that include compressors, bypass fans, and turbines. Typically, air is forced into the engine and passed into a compressor. The compressed air is passed to a combustor, and at high temperature and pressure the combustion products are passed into a turbine. The turbine provides power to the shaft, which in turn provides the power to the compressor and bypass fan or gearbox. Thrust is thereby produced from the air that passes from the bypass fan, as well as from the thrust expended in the turbine combustion products. This system is typically packaged together with power production and thrust generation co-located.
However, air can be thermodynamically inefficient, especially during high altitude operation of the engine (such as in an aircraft application). Air that enters the engine is of low pressure, therefore low density. In order to reach the needed pressure and temperature at the combustor exit, the air is compressed to very high pressure ratios and heated up to very high temperatures in the combustors. In order to provide adequate mass flow rate, significant volume flow rate of the low density air is pumped through high pressure ratio consuming significant amount of power. As a result the engines are made of large and heavy components, consume large amount to fuel, and may include significant operational and maintenance expenses to cope with high combustion temperatures.
To increase system efficiency and reduce component size and complexity of turbomachinery, some power-producing or thrust-producing use a closed cycle super-critical carbon dioxide (s-CO2) system. This system provides significantly improved efficiencies compared to Brayton and other air-based systems by operating in a super-critical region (operating at a temperature and pressure that exceed the critical point). That is, a phase-diagram of CO2, as is commonly known, includes a “triple point” as the point that defines the temperature and pressure where solid, liquid, and vapor meet. The critical point is the top of the dome made up of the saturated liquid and saturated vapor lines. Above the critical point is the gaseous region. At the triple point the fluid can exist in liquid, vapor, or in a mixture of the both states. However, at higher temperature and pressure, a critical point is reached which defines a temperature and pressure where gas, liquid, and a super-critical region occur.
Fluids have a triple point, a critical point, saturated liquid and vapor lines, and a super-critical region. One in particular, carbon dioxide, is particularly attractive for such operation due to its critical temperature and pressure of approximately 31° C. and 73 atmospheres, respectively, as well as due to its lack of toxicity. Thus, s-CO2-based systems may be operated having very dense super-critical properties, such as approximately 460 kg/m3. The excellent combination of the thermodynamic properties of carbon dioxide may result in improved overall thermodynamic efficiency and therefore a tremendously reduced system size. Due to the compact nature and high power density of a power source that is powered with a super-critical cycle, the overall size of engine may be significantly reduced, as well.
Aircraft typically include one or two propulsion systems placed on each wing or integrated the aft fuselage or tail. Their placement is based at least in part on the size of the thrust-producing devices, such as gas turbine engines, in which air is traditionally used as the working fluid. However, typical propulsion systems are sized to account for a mis-match between the LP shaft speed of the engine and the desired speed of the propulsor. Not only does this result in conversion inefficiencies, but additional mass as well. Distributed propulsion is a method which addresses this sizing mismatch by decoupling the propulsor from the power shaft and integrating it more closely with the air vehicle for aerodynamic improvement. This also allows for higher bypass ratios, and air vehicle lift or drag augmentation.
As such, there is a need to improve overall propulsion efficiencies and reduce mass employing a s-CO2 operation.